An alumina-based sintered body comprising alumina as a main component has excellent withstand voltage characteristics, heat resistance and mechanical strength, and is inexpensive. Therefore the alumina-based sintered body is used as ceramic products such as an insulator of a spark plug, and a multilayer wiring board of IC package. The alumina-based sintered body was formed by sintering a mixed powder containing a sintering aid such as a three-component sintering aid comprising SiO2—CaO—MgO. For example, Patent Document 1 describes a method for manufacturing a high insulating high alumina porcelain composition, which comprises molding and burning a mixed raw material powder includes at least one additive selected from Y2O3, ZrO2 and La2O3, or a solid solution composite oxide of at least one additive selected from Y2O3, ZrO2 and La2O3, and alumina, and alumina fine particle powder having a particle diameter of 0.5 μm or less, thereby preparing a sintered body, the content of the additive being 0.5 to 10 wt % to the sintered body.
Patent Document 2 describes “an alumina porcelain constituted of a sintered body having a porosity of 6% by volume or less, the sintered body comprising alumina (Al2O3) having an average particle diameter of 1 μm or less, and at least one of compound and mixture of at least one of yttria (Y2O3), magnesia (MgO), zirconia (ZrO2) and lanthanum oxide (La2O3), formed in grain boundary and Al2O3.”
However, in the case of forming an insulator for the spark plug by using the alumina-based sintered body described above, the sintering aid (mainly Si component) is present as a low melting point glass phase in grain boundary of alumina crystal particles after sintering. Therefore, under a usage environment of the spark plug, for example, a high temperature environment at about 700° C., the low melting point glass phase softens, and withstanding voltage characteristics of the insulator are decreased. On the other hand, a low melting point glass phase in an alumina-based sintered body can be decreased by decreasing the amount of the sintering aid added. In this case, an insulator is not densified, or even though densified seemingly, many pores remain in grain boundary constituted by alumina crystal particles, and withstanding voltage characteristics of the insulator are decreased.
The conventional alumina-based sintered body has a low melting glass phase or pores (residual pores) present in grain boundary. In the case of forming an insulator of a spark plug with such an alumina-based sintered body, when high voltage for generating spark discharge is applied to a spark plug in high temperature environment of about 700° C., a low melting point glass phase softens, or electric field concentrates in residual pores, and an insulator may suffer breakdown (spark penetration).
An insulator or a material thereof for a spark plug is proposed for the purpose of preventing decrease in withstand voltage characteristics and/or breakdown. For example, Patent Document 3 describes “an alumina-based sintered body containing at least a rare earth element (hereinafter referred to as “RE”) component, the alumina-based sintered body having a theoretical density ratio of 95% or more.”
Patent Document 4 describes “an insulator for a spark plug, in which the content ratio in terms of oxide of Al component is 95 to 99.8 mass % when the sum of the constituent components is defined as 100 mass %, which contains a rare earth element and Si component in amounts such that a ratio (RRE/RSi) of a content ratio in terms of oxide of the rare earth element (RRE) to a content ratio in terms of oxide of the Si component (RSi) is 0.1 to 1.0, in which the number of alumina particles having the maximum length present on a cut surface of 1 mm2 of 10 μm or more and an aspect ratio of 3 or more is less than 10.”
Patent Document 5 describes “an alumina porcelain composition using alumina as a main component, comprising a composite sintered body of the alumina as the main component, and a composition of at least one element selected from Al, Si, Mg and rare earth elements, wherein when the amount of alumina as the main component is 100 parts by weight, the amount of the composition of at least one element selected from Al, Si, Mg and rare earth elements is 5 parts by weight or less.”
In recent years, increase in occupation space of inlet and exhaust valves in a combustion chamber and 4-valve formation is investigated in internal combustion engines having a spark plug mounted thereon with increase in power of internal combustion engines. For this reason, a spark plug itself and its insulator tend to be reduced-sized (small diameter) and to decrease its thickness. Therefore, an insulator having decreased thickness is required to have high mechanical strength in high temperature environment of about 700° C. in addition to prevention of decrease in withstand voltage characteristics and breakdown. However, the insulators of spark plug or their materials described in Patent Documents 3 to 5 are not investigated on mechanical strength in high temperature environment (hereinafter referred to as “strength at high temperature”).
On the other hand, an alumina-based sintered body constituting an insulator of a spark plug is generally prepared by compression molding a raw material powder prepared to obtain a molded article (hereinafter referred to as an “unburned molded article”), grinding and shaping the unburned molded article into a desired shape and a desired thickness, and then burning the same. Therefore, an alumina-based sintered body forming an insulator of a spark plug is required to have high processability, particularly grinding processability, for enabling to decrease thickness, in addition to the above excellent characteristics when forming an insulator. In particular, recent insulators have a small thickness. Therefore, an unburned molded article becoming an alumina-based sintered body is required to have excellent processability, particularly grinding processability. The reason for this is that where an unburned molded article has poor processability and its processing time is prolonged, productivity of an alumina-based sintered body and thus a spark plug is decreased.
To achieve high productivity of an alumina-based sintered body by improving processability of an unburned molded article, it is effective to use a raw material powder having a large particle diameter such that an alumina-based sintered body has an average crystal particle diameter of 1.50 μm or more. However, a raw material powder having a large particle diameter has low sinterability, and an alumina-based sintered body obtained by burning such a raw material powder cannot sufficiently be satisfied with the characteristics, particularly mechanical strength, required as an insulator of a spark plug in some cases.
Thus, in the present situation that processability of an unburned molded article and the characteristics, particularly mechanical strength, as a sintered body have a contradictory relationship, the characteristics, particularly mechanical strength, when forming an insulator are considered important, and processability of an unburned molded article capable of forming an insulator is not investigated. Thus, it has not been easy to form an insulator having desired shape and thickness by an industrially inexpensive method.    Patent Document 1: JP-B-7-17436    Patent Document 2: JP-B-7-12969    Patent Document 3: JP-A-2001-2464    Patent Document 4: JP-A-2001-335360    Patent Document 5: WO 05/033041 pamphlet